When a bill (i.e., bank note or paper currency) is inserted or entered by a customer into an automatic cash deposit/dispensation machine or an automatic vending machine, the denomination of the bill is identified and the authenticity of the bill is tested. For this reason, a bill examination device is used.
The bill examination device detects a magnetic pattern or an optical pattern of reflected light or transmitted light of a bill that has been entered, compares it with a preset reference pattern, and, on the basis of the similarity with the reference pattern, tests the authenticity of the bills that has been entered into the machine.
A problem associated with the prior-art bill examination device described above is the similarity test is easily affected by differences or variations in the sensitivities or offsets of the detecting elements and other circuit components due to manufacturing inaccuracies, due to temperature change or due to aging (change with lapse of time), or differences in the darkness of printing of the bill or smudge on the bill, and if the required level of similarity is lowered to allow for the variations, a forged bill may erroneously be found as an authentic bill. Thus, the reliability of the authenticity test is degraded.